Various types of magnetic field sensing elements are known, including Hall Effect elements and magnetoresistance elements. Magnetic field sensors generally include a magnetic field sensing element and other electronic components. Some magnetic field sensors also include a fixed permanent magnet.
Magnetic field sensors provide an electrical signal representative of a sensed magnetic field, or, in some embodiments, fluctuations of a magnetic field associated with the permanent magnet. In the presence of a moving ferromagnetic object (e.g., a gear, a ring magnet, or a linear multi-pole magnet), the magnetic field sensed by the magnetic field sensor can vary in accordance with a shape or profile of the moving ferromagnetic object.
Magnetic field sensors are often used to detect movement of features of a ferromagnetic gear, such as gear teeth and/or gear slots. A magnetic field sensor in this application is commonly referred to as a “gear tooth” sensor.
In some arrangements, the gear is placed upon a target object, for example, a camshaft in an engine, thus the rotation of the target object (e.g., camshaft) is sensed by detection of the moving features of the gear. Gear tooth sensors can be used in automotive applications, for example, to provide information to an engine control processor for ignition timing control, fuel management, and other operations.
Information provided by the gear tooth sensor to the engine control processor can include, but is not limited to, an absolute angle of rotation of a target object (e.g., a camshaft) as it rotates, and a direction of rotation. With this information, the engine control processor can adjust the timing of firing of the ignition system and the timing of fuel injection by the fuel injection system. Thus, it will be recognized that accurate information about the angle of rotation is important for proper engine operation.
Gear tooth sensors can also be used in other applications, including, but not limited to, anti-lock braking systems and in transmissions.
Many types of magnetic field sensors may not provide an accurate output signal (e.g., indication of absolute angle of rotation of an object) immediately upon power up of the magnetic field sensor, and/or immediately upon movement of the target object from zero rotating speed, and/or upon movement slowing to zero rotating speed, but instead provide an accurate output signal only once the magnetic field sensor has been powered up for a period of time, and the target object has moved through a substantial rotation or is moving with substantial speed.
The above accurate output signal refers to accuracy of the positions of transition edges (corresponding to gear teeth edges, ring magnet pole edges, or linear multi-pole magnet pole edges) in the output signal from the magnetic field sensor.
In general, a so-called “precision” rotation detector can provide an accurate output signal only after some period following power up of the magnetic field sensor and after the gear has been rotating for some period of time. In contrast, in general, a so-called “true power on state” (TPOS) detector can provide a reasonably accurate output signal, but less accurate than the precision rotation detector, at an earlier time after the magnetic field sensor powers up and at an earlier time after the gear starts rotating.
One type of precision rotation detector is described in U.S. Pat. No. 6,525,531, issued Feb. 25, 2003. This precision rotation detector includes a positive digital-to-analog converter (PDAC) and a negative digital-to-analog converter (NDAC) that track positive and negative peaks of magnetic field signal, respectively, for use in generating a threshold signal. A varying magnetic field signal is compared to the threshold signal. However, the outputs of the PDAC and the NDAC may not provide accurate indications of the positive and negative peaks of the magnetic field signal until several cycles of the signal (i.e., signal peaks) occur (i.e., until several gear teeth have passed). Other types of precision rotation detectors are described, for example, in U.S. Pat. No. 7,199,579, issued Apr. 2, 2007, U.S. Pat. No. 7,368,904, issued Apr. 6, 2008, U.S. Pat. No. 6,297,627, issued Oct. 2, 2001, and U.S. Pat. No. 5,917,320, issued Jun. 29, 1999, each of which is incorporated herein by reference, and each of which is assigned to the assignee of the present invention.
A conventional TPOS detector is described in U.S. Pat. No. 7,362,094, issued Apr. 22, 2008. Some conventional TPOS detectors simply compare a magnetic field signal with a fixed, predetermined, and sometimes trimmed, threshold.
A TPOS detector can be used in conjunction with a precision rotation detector, both providing information to the engine control processor. A TPOS detector can be combined in the same integrated circuit with a precision rotation detector, and the magnetic field sensor after power up, can first use the TPOS detector and then switch to use the precision rotation detector.
As described above, the conventional TPOS detector provides a reasonably accurate output signal with only a small initial rotation of the target object, and before the precision rotation detector can provide an accurate output signal. Furthermore, the TPOS detector can provide information about whether it is proximate to a gear tooth or a gear valley nearly immediately upon power up.
A TPOS detector can provide information to the engine control processor that can be more accurate than information provided by the precision rotation detector at times proximate to the time of power up of the magnetic field sensor, and also at times near the beginning and end of rotation of the target object (e.g., start and stop of the engine and camshaft). However, the TPOS detector may be less accurate than the precision rotation detector at some time after the magnetic field sensor has powered up and when the object is rotating at full speed. When the object is rotating at full speed, the engine control processor can primarily use rotation information provided by the precision rotation detector.
As described above, unlike the precision rotation detector, the conventional TPOS detector has a fixed predetermined threshold against which a magnetic field signal is compared. An output signal from a conventional TPOS detector has two states, typically a high and a low state, in accordance with features on the target object.
It is known that various parameters affect a magnitude of the magnetic field signal generated by the TPOS magnetic field sensor. For example, temperature is known to affect a sensitivity of the Hall element, and therefore, a magnitude of the magnetic field signal. Changes in size of an air gap between the TPOS magnetic field sensor and the TPOS cam or gear can also affect a magnitude of the magnetic field signal.
In a TPOS detector, when the magnetic field signal is compared against a fixed predetermined threshold, the changes in the amplitude of the magnetic field signal due to temperature, air gap, etc., can cause undesirable changes in positions of the edges in the above-described output signal from the TPOS detector.
A conventional TPOS detector can have a comparator therein operating as a comparator detector.
It would be desirable to provide TPOS detector, or more generally, a comparator detector, for which positions of edges of an output signal therefrom vary less than for a conventional TPOS or comparator detector.